Method of Manufacturing Semiconductor Device and Semiconductor Device

ABSTRACT

A method of manufacturing a semiconductor device, in which a second semiconductor layer of Al x Ga 1-x-y In y N (wherein x, y, and x+y satisfy x&gt;0, y≧0, and x+y≦1, respectively) on a first semiconductor layer of GaN by hetero-epitaxial growth using a MOCVD method, the method including the steps of: (a) supplying N source gas and Ga source gas to form the first semiconductor layer; (b) supplying the N source gas without supplying the Ga source gas and Al source gas, after step (a); (c) supplying the N source gas and the Al source gas without supplying the Ga source gas, after step (b); and (d) supplying the N source gas, the Ga source gas and the Al source gas to form the second semiconductor layer, after step (c).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No.2010-266573 filed on Nov. 30, 2010, the entire subject matter of whichis incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to a method of manufacturing a semiconductordevice using an abrupt heterojunction interface of a compoundsemiconductor. Further, this disclosure also relates to thesemiconductor device.

BACKGROUND

A high electron mobility transistors (HEMT) using, for example, GaN hasbeen used as semiconductor devices using compound semiconductors,specifically as high-power/high-frequency elements. FIG. 5 illustratesan outline of a cross-sectional structure of a HEMT device(semiconductor device) 10 using a nitride semiconductor. In FIG. 5, on asubstrate 11, an electron transit layer 12 and a barrier layer 13 aresequentially formed by epitaxial growth. Herein, for example, theelectron transit layer 12 is made of semi-insulating (undoped) GaN, andthe barrier layer 13 is made of semi-insulating (undoped) AlGaN(precisely semi-insulating (undoped) Al_(x)Ga_(1-x)N, where x is about0.20). In this structure, on a side of the electron transit layer 12 inthe vicinity of an interface (hetero interface) of the electron transitlayer 12 and the barrier layer 13, a two-dimensional electron gas layer14 (shown by a broken line in FIG. 5) that is to be an electricallyconductive layer is formed in parallel with the hetero interface by apiezoelectric effect. According to the two-dimensional electron gaslayer 14, a current flow between a source electrode 15 and a drainelectrode 16, and a channel configured by the two-dimensional electrongas is switched on or off according to a voltage applied to a gateelectrode 17 that is to be a Schottky electrode. At this time, since thespeed (mobility) of electrons in the two-dimensional electron gas isvery high, a high-speed operation is possible. Further, since GaN has aband gap wider than of that of GaAs or the like, the HEMT device 10 hasa high pressure resistance and thus is capable of a high-poweroperation.

In the HEMT device 10, in order to increase the mobility in thetwo-dimensional electron gas layer 14 and obtain high conductance, it isrequired that the interface (hetero interface) of the electron transitlayer 12 and the barrier layer 13 is abrupt, that is, a compositionvariation between the electron transit layer 12 and the barrier layer 13is abrupt at the interface. The electron transit layer 12 and thebarrier layer 13 are consecutively formed by, for example, a metalorganic chemical vapor deposition (MOCVD) method, a molecular beamepitaxy (MBE), or the like.

In order to obtain a HEMT device having high mobility, for example,JP-A-2004-200711 discloses a technology of inserting a spacer layer,which has high aluminum composition and a wide band gap, in the vicinityof a hetero interface. FIG. 6 is a cross-sectional view illustrating across-sectional structure of a HEMT device (semiconductor device) 30disclosed in JP-A-2004-200711. Here, between an electron transit layer(GaN layer) 12 and a barrier layer (AlGaN layer) 13, an AlN spacer layer20 is as thin as one to four molecular layers (about 0.25 nm to 1 nm) isinserted. It is possible to widen the band gap at a position of thespacer layer 20 and to improve abruptness in a band structure, therebysubstantially improving abruptness at the hetero interface. According tothis technology, it is possible to increase the mobility at thetwo-dimensional electron gas layer 14 and to obtain a HEMT device havinghigh conductance.

SUMMARY

However, even in the case of using the structure disclosed inJP-A-2004-200711, it is difficult in a heterojunction of a nitridesemiconductor to obtain actually an ideally abrupt hetero interface inan electron transit layer, a spacer layer, and a barrier layer by usingthe MOCVD method. For this reason, it is difficult to obtain ahigh-performance HEMT device using a nitride semiconductor.

According to this disclosure, a semiconductor device, which has anabrupt hetero interface formed therein, made of a nitride semiconductorhaving high mobility is provided.

A method of manufacturing a semiconductor device in one aspect of thisdisclosure, in which a second semiconductor layer ofAl_(x)Ga_(1-x-y)In_(y)N (wherein x, y, and x+y satisfy x>0, y≧0, andx+y≦1, respectively) on a first semiconductor layer of GaN byhetero-epitaxial growth using a MOCVD method, the method comprising thesteps of: (a) supplying N source gas and Ga source gas to form the firstsemiconductor layer; (b) supplying the N source gas without supplyingthe Ga source gas and Al source gas, after step (a); (c) supplying the Nsource gas and the Al source gas without supplying the Ga source gas,after step (b); and (d) supplying the N source gas, the Ga source gasand the Al source gas to form the second semiconductor layer, after step(c).

Since this disclosure is configured as described above, it is possibleto provide a semiconductor device, which has an abrupt hetero interfaceformed therein, made of a nitride semiconductor having high mobility.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and additional features and characteristics of thisdisclosure will become more apparent from the following detaileddescriptions considered with the reference to the accompanying drawings,wherein:

FIG. 1 is a view illustrating a source-gas supplying situation in eachprocess of a method of manufacturing a semiconductor device according toan exemplary embodiment of this disclosure;

FIG. 2 illustrates a result of measurement of a composition distributionin the vicinity of an interface when forming an MN spacer layer directlyon a GaN layer according to the background art;

FIG. 3 illustrates a result of measurement of a composition distributionin the vicinity of an interface when forming an AlGa layer on a GaNlayer by using the method of manufacturing the semiconductor deviceaccording to the exemplary embodiment of this disclosure;

FIG. 4 is a view illustrating a source-gas supply situation in eachprocess of a modification of the method of manufacturing a semiconductordevice according to the exemplary embodiment of this disclosure;

FIG. 5 is a cross-sectional view of one example of HEMT device; and

FIG. 6 is a cross-sectional view of another example of the HEMT deviceaccording to the background art.

DETAILED DESCRIPTION

Hereinafter, a method of manufacturing a semiconductor device accordingto an exemplary embodiment of this disclosure will be described. Thesemiconductor device manufactured herein is a high electron mobilitytransistor (HEMT) element using a nitride semiconductor. In this HEMTdevice, a first semiconductor layer (electron transit layer: GaN) and asecond semiconductor layer (barrier layer: AlGaN) are sequentiallyformed on a substrate. A current flow in a direction along a heterointerface between the first semiconductor layer and the secondsemiconductor layer, that is, in a direction parallel with the substrateplane, and the HEMT device is operated.

A cross-sectional structure of the semiconductor device, which ismanufactured by this method, is similarly to FIG. 5. In other words, thesemiconductor device 10 includes an electron transit layer (firstsemiconductor layer) 12 and a barrier layer (second semiconductor layer)13 sequentially formed on a substrate 11. But, the semiconductor device10 does not include a spacer layer 20 as shown in FIG. 6. The substrate11 is monocrystal of silicon (Si) or silicon carbide (SiC), for example.The electron transit layer 12 is made of semi-insulating (undoped) GaN,and the barrier layer 13 is made of semiconductor (undoped) AlGaN(precisely, semi-insulating (undoped) Al_(x)Ga_(1-x)N, where x≦0.5). Themethod of manufacturing a semiconductor device described hereinspecifically relates to a growth method from the electron transit layer12 to the barrier layer 13. This growth method is based on the MOCVDmethod.

In the MOCVD method, source gases are used for growing GaN and/or AlGaN.Trimethylgallium (TMG) is used as Ga source, ammonia (NH₃) is used as anitrogen (N) source, and trimethylaluminum (TMA) is used as an Alsource. Further, as carrier gas for carrying those source gases,hydrogen (H₂) or the like is used. A substrate temperature during thegrowth is about 1100° C. As a plane orientation of the substrate 11, ina case of Si, a (111) Miller plane is preferably used. In a case of SiC,a (0001) Miller plane is preferably used. In this case, theabove-mentioned gases are flowed on the substrate 11, thereby growing ac-plane of GaN or AlGaN having a wurtzite crystal structure on thesubstrate 11. In this structure, both of the electron transit layer 12and the barrier layer 13 are undoped, so that gas for doping is notused. Incidentally, as will be described below, it is possible to makethe barrier layer (second semiconductor layer) 13 ofAl_(x)Ga_(1-x-y)In_(y)N (where x>0, y≧0, and x+y<1). In this case,indium (In) will be added and indium (In) source gas will be also used.

FIG. 1 is a view schematically illustrating a situation of supply (ON)and supply disruption (OFF) of the above-mentioned gases from the growthof the electron transit layer 12 (GaN) to the growth of the barrierlayer 13 (AlGaN), changing with time in the growth method. The processesare broadly divided into a first growth process, a growth interruptprocess, a pre-flow process, and a second growth process. Actually, aflow rate of each gas is appropriately controlled; however, the flowrate of each gas is simplified. Additionally, the carrier gas and thenitrogen source gas (NH₃) are supplied (ON) over all processes.

First, in the first growth process, in order to grow the electrontransit layer 12 (GaN layer), NH₃ and TMG are supplied (ON) to a chamberof a MOCVD apparatus, and TMA is not supplied (OFF). In the first growthprocess, the flow rate of TMG is set to a first flow rate f₁, and a timeperiod t₁ of the first growth process is appropriately set according tothe thickness of the electron transit layer 12 (GaN layer).

Next, prior to the growth of the barrier layer 13, the growth interruptprocess, in which both of TMG and TMA are not supplied, is performedduring a time period t₂. This time period t₂ is set as a certain amountof time so that Ga source gas remains in the chamber. For example, thetime period t₂ is set to 5 minutes or less, more preferably 1 minute orless, for the following reason. Incidentally, the time period t₂ may beset to, for example about 20 seconds, according to the processconditions such as the flow rate of the carrier gas (H₂) or the nitrogensource gas (NH₃).

Next, the pre-flow process, in which TMA is supplied (ON) and but TMG isnot supplied (OFF), is performed. In the pre-flow process, the flow rateof TMA is set to a second flow rate f₂. The second flow rate f₂ is setso that an abrupt interface is formed between the electron transit layer12 and the barrier layer 13. A time period t₃ of the pre-flow process isshorter than a period when the AlGaN layer is formed so as to be as thinas two molecular layers (one molecular layer has a thickness of about0.25 nm).

Next, the second growth process, in which TMG and TMA are supplied, forgrowing the barrier layer 13 (AlGaN layer) is performed. In the secondgrowth process, the flow rate of TMG is set to a third flow rate f₃lower than the first flow rate f₁, and the flow rate of TMA is set to afourth flow rate f₄. The third flow rate f₃ and the fourth flow rate f₄are appropriately set so that the barrier layer 13 having a desiredcomposition is to be formed. For example, (f₃ is smaller than f₁) and(f₄ is subequal to f₂) can be satisfied. A time period t₄ of the secondgrowth process is appropriately set according to the thickness of thegrown barrier layer 13 (AlGaN). After that, the supply of TMA and TMG isshut off, thereby finishing the growth.

In the above processes, since the growth interrupt process and thepre-flow process are provided, it is possible to improve the abruptnessat the hetero interface between the electron transit layer 12 and thebarrier layer 13. This reason will be described below.

FIG. 2 illustrates a result of an energy dispersive X-ray spectroscopy(EDX) measurement of a depth distribution of an Al composition in asection of a sample, in which an AlN layer (spacer layer 20) is directlygrown on a GaN layer. FIG. 2 corresponds to a case of manufacturing thestructure, which is disclosed in JP-A-2004-200711 and is shown in FIG.6. In this case, immediately after the first growth process(electron-transit-layer growth process), TMA is supplied (ON) for a longtime (TMG is not supplied (OFF) for a long time) to grow the AlN layer(spacer layer 20), without performing the growth interrupt process ofFIG. 1. This time period is set to a time period in which the AlN layeris grown about 5 nm. In FIG. 2, the left side corresponds to a side ofthe electron transit layer 12 (a side of substrate 11), and the rightside corresponds to a side of the barrier layer 13. In FIG. 2, a brokenline (A) illustrates a designed interface between the electron transitlayer 12 and the spacer layer 20, and a broken line (B) is a designedinterface between the spacer layer 20 and the barrier layer 13. In otherwords, the supply of TMG is shut off and the supply of TMA starts at atime point when an area indicated by the broken line (A) of FIG. 2 isgrown, and the supply of TMG restarts at a time point when an areaindicated by the broken line (b) is grown.

From this result, it can be seen that even if the above-mentioned growthis performed on a condition so that AlN is grown on GaN, actually, thespacer layer 20 composed of AlN (having the Al composition of 100%) isnot formed. Further, since the Al composition gently changes from theelectron transit layer 12 to the barrier layer 13, it can also be seenthat an abrupt junction is not obtained.

The cause is that, even if the growth of the spacer layer 20 (AlN layer)starts in a state in which TMG is not shut off, TMG used for the growthof the electron transit layer 12 (GaN layer) remains at a relativelyhigh concentration in a growth atmosphere around the substrate disposedin the chamber. Further, in connection with this point, even when TMA issupplied, the effect is appeared with being delay. In other words, in acase where TMA is supplied for a short time, AlN is not grown. Actually,when TMA is supplied for a time period corresponding to the thickness of5 nm, the AlN layer is not obtained.

Thus, in order to grow the barrier layer 13 (AlGaN layer) on theelectron transit layer 12 (GaN layer) so that the composition changesabruptly, at least, it is preferable to reduce the concentration of TMGremaining in the chamber prior to the growth of AlGaN, and it ispreferable to provide the effect of TMA earlier than in the time periodof the growth of the AlGaN layer. Therefore, in the growth method ofFIG. 1, since the growth interrupt process and the pre-flow process areapplied, the abruptness is improved. It is preferable that a total flowrate of TMA in the pre-flow process is set so that the AlGaN layer isgrown to be as thin as two molecular layers or less.

FIG. 3 illustrates a result of measurement of a composition distributionin the vicinity of an interface between the electron transit layer 12and the barrier layer 13 obtained by the growth method of FIG. 1. FIG. 3illustrates a secondary ion mass spectroscopy (SIMS) analysis result ofa case of performing sputter etching from the grown surface. Unlike FIG.2, the left side in FIG. 3 corresponds to the surface side (a side ofbarrier layer 13), a vertical axis represents a logarithmic scale, and abroken line (C) represents a designed interface between the electrontransit layer 12 and the barrier layer 13. As a result of the growthmethod shown in FIG. 1, it can be seen that the composition of Al and Gachanges in the vicinity of the designed interface, and an abruptinterface is obtained between the electron transit layer 12 and thebarrier layer 13. Further, although only TMA is supplied (ON) in thepre-flow process, the AlN layer (specifically, a layer having a high Alcomposition) is not formed at the interface. Thus, an ideally abruptinterface between the GaN layer and the AlGaN layer is obtained.

As described above, according to the above-mentioned manufacturingmethod, it is possible to improve the abruptness at the interfacebetween the GaN layer and the AlGaN layer. Accordingly, it is possibleto improve the mobility in the two-dimensional electron gas layer 14 andto improve the conductance, as described in the technology disclosed inJP-A-2004-200711.

Additionally, in a case of inserting the spacer layer 20 having a wideband gap as disclosed in JP-A-2004-200711, the ON resistance is reducedand but the leakage current increases. Further, in a case of anelectrode structure of FIG. 6 (a structure in which the source electrode15 and the drain electrode 16 are disposed on the surface), the spacerlayer 20 having a wide band gap and high electrical resistance isinserted between the electrodes and the two-dimensional electron gaslayer 14. Therefore, the contact resistance increases between the sourceelectrode 15 or the drain electrode 16 and the two-dimensional electrongas layer 14.

In contrast, according to the manufacturing method of the exemplaryembodiment of this disclosure, it is not required to provided a layerhaving a wide band gap (a layer having a high Al composition) such asthe AlN layer. In other words, for example, even if the Al compositionratio of the barrier layer 13 (a value of x in Al_(x)Ga_(1-x)N) is 0.5or less, since an abrupt interface is formed between the barrier layer13 and the electron transit layer 12, the above-mentioned problems inleakage current and contact resistance are improved. With consideringthese points, in the above-mentioned manufacturing method, it isrequired to set the time period t₂ of the growth interrupt process sothat TMG remains in the chamber in starting the pre-flow process.Preferably, the time period t₂ of the growth interrupt process is set sothat the concentration of TMG in starting the pre-flow process is thesame as or is lower than the concentration of TMG in the second growthprocess. Therefore, it is possible to improve the mobility in thetwo-dimensional electron gas layer 14 and obtain preferablecharacteristics as a HEMT device, without increasing the leakage currentand increasing the resistance of the source electrode or the drainelectrode.

Further, if it is to form a spacer layer having a high Al compositionratio or a layer equivalent to the spacer layer, it is preferable tolengthen the time period t₂ of the growth interrupt process and todischarge TMG from the chamber. However, for example, in a case ofperforming the growth interrupt process for five minutes or more so asto form a HEMT device, the characteristic deterioration such as anincrease in leakage current or current collapse occurs. It would appearthat, when the concentration of TMG in the chamber is to be almost 0during the growth interrupt process, the surface of the already formedelectron transit layer 12 may get rough due to the carrier gas or NH₃flowing in the growth interrupt process. For this reason, the flatnessof the surface may be damaged. In the above-mentioned manufacturingmethod, since the time period t₂ of the growth interrupt process is setto 1 minute or less, that characteristic deterioration of the HEMTdevice is suppressed.

As a modification of the above-mentioned manufacturing method (growthmethod), a case of making the barrier layer 13 of AlInN (Al_(x)In_(y)Nwhere x+y=1), not AlGaN, will be described. FIG. 4 is a viewschematically illustrating a situation of supply (ON) and supplydisruption (OFF) of each gas with time in this modified case. In thiscase, as a source of indium (In) of FIG. 1, trimethylindium (TMI) isused. Therefore, in the second growth process, the TMA and TMI aresupplied (ON).

However, in the pre-flow process, TMI is not supplied (OFF), and onlyTMA is supplied (ON). As described above, this is for the abruptness ofthe Ga composition and the Al composition. In other words, even when thebarrier layer 13 is made of AlInN, if only the nitrogen source gas andthe Al source gas (and the carrier gas) are supplied in the pre-flowprocess, it is possible to improve the abruptness at the heterointerface between the GaN layer and the AlInN layer.

Even when the barrier layer 13 is made of Al_(x)Ga_(1-x-y)In_(y)N (wherex>0, y≧0, and x+y≦1) which is intermediate between AlGaN and AlInN, thesame effect is obtained. Even in this case, x≦0.5 is preferable, asdescribed above.

Incidentally, although the barrier layer (second semiconductor layer) isundoped in the above-mentioned example, it is apparent that theabove-mentioned manufacturing method can be applied to even a case wherethe second semiconductor layer is doped with an impurity. In this case,the source gas of the impurity may be supplied (ON) only in the secondgrowth process, and the supply of the source gas of the impurity may benot supplied (OFF) in processes prior to the second growth process.

Further, the flow rate of each source gas is appropriately set accordingto the composition and thickness of each layer to be grown. It is alsopossible to set an appropriate standby time during switching to each ofthe above-mentioned processes for the growth, that is, between theprocesses.

Furthermore, in the above-mentioned example, the HEMT device has beendescribed. However, it is apparent that the above-mentionedmanufacturing method is effective for a semiconductor device uses aheterojunction between a GaN layer and an AlGaN layer (or AlInN layer),such as a diode element or a combined element using that and a diodeelement which.

1. A method of manufacturing a semiconductor device, in which a secondsemiconductor layer of Al_(x)Ga_(1-x-y)In_(y)N (wherein x, y, and x+ysatisfy x>0, y≧0, and x+y≦1, respectively) on a first semiconductorlayer of GaN by hetero-epitaxial growth using a MOCVD method, the methodcomprising the steps of: (a) supplying N source gas and Ga source gas toform the first semiconductor layer; (b) supplying the N source gaswithout supplying the Ga source gas and Al source gas, after step (a);(c) supplying the N source gas and the Al source gas without supplyingthe Ga source gas, after step (b); and (d) supplying the N source gas,the Ga source gas and the Al source gas to form the second semiconductorlayer, after step (c).
 2. The method of manufacturing a semiconductordevice according to claim 1, wherein the Ga source gas remains aroundthe first semiconductor layer in starting step (c), and wherein thesecond semiconductor layer is made of Al_(x)Ga_(1-x-y)In_(y)N (whereinx, y, and x+y satisfy x>0, y≧0, and x+y<1, respectively).
 3. The methodof manufacturing a semiconductor device according to claim 2, whereinthe second semiconductor layer is made of Al_(x)Ga_(1-x-y)In_(y)N (x, y,and x+y satisfy 0<x≦0.5, y≧0, and x+y<1, respectively).
 4. The method ofmanufacturing a semiconductor device according to claim 1, wherein atime period of step (b) is set to 5 minutes or less.
 5. The method ofmanufacturing a semiconductor device according to claim 1, wherein theGa source gas is trimethylgallium (TMG), the N source gas is NH₃(ammonia), and the Al source gas is trimethylaluminum (TMA).
 6. Asemiconductor device manufactured by the method of manufacturing asemiconductor device according to claim 1.